1. Field of the Invention
This invention relates to improved catalytic reaction systems and to methods for the preparation of catalysts.
2. Brief Description of Related Art
Automotive emissions are a major environmental problem in spite of the advances brought about by the use of catalytic converters. One factor limiting the performance of catalytic converters is that pollution is not controlled during the thirty or so seconds required to bring the converter catalyst to its operating temperature. In present converters, warm-up is dependent on heating of the catalyst by hot engine exhaust gases. Although electrical heating could be utilized to preheat the catalyst prior to engine operation, the power and the time delay required with present catalyst structures, ceramic or metal, have been deemed unacceptable.
Subsequent to catalyst light-off, surface reactions on conventional monolithic catalysts such as are used in catalytic converters are mass transfer limited. Thus, the catalyst mass required for a given conversion level is much higher than if no mass transfer limitation existed at the given operating conditions. The high catalyst mass required for the required conversion level results in the relatively long heat-up times experienced, even with electrical heating. In addition, this mass transfer limitation is such that the conversion level of present automotive exhaust catalytic converters is limited to relatively low levels, typically not more than about 95%, even with the relatively small catalyst channel sizes employed. Higher conversion levels would be advantageous.
The need to reduce catalyst warm-up time of the conventional ceramic monolith automotive catalysts to reduce emissions during the warm-up period has led to increased interest in metal monolith catalysts. However, merely substituting metal for ceramic in a conventional monolith structure yields catalysts which still have much too high a thermal mass. Although metal monoliths are electrically conductive and could therefore be electrically preheated, fast enough heat up times have not yet been demonstrated as feasible. Furthermore, thermal shock damage would likely be a problem if a conventional metal monolith were heated as rapidly as needed for elimination of start-up emissions. There is a critical need for a catalyst system which can control hydrocarbon emissions during initial engine operation.
For catalytic combustors the problem is not just emissions but the ability to function in certain applications. For example, an automotive catalytic combustor gas turbine must start in roughly the same time frame as present automotive engines.
The present invention provides catalysts and systems which make possible much more rapid warm-up of converter catalysts without electrical heating and near instantaneous electrical heating of catalysts in combustors and catalytic converters. Moreover, catalysts of the present invention enable much higher conversions and improved selectivity in many chemical conversion processes by virtue of improved mass transfer to and from the catalyst surface. The process of the invention provides catalyst articles of improved durability, efficiency and service life.